It is known that when a fluorochemical surfactant is added into an inkjet recording ink (hereinafter referred to simply as “ink”), the surface tension of the ink can be reduced, color material (colorant) components easily remain on the surface of a recording medium such as paper (because vehicles of the ink quickly permeate through the recording medium), and thus there is an improvement in image density and image properties.
However, when a conventional fluorochemical surfactant is used, there exists the following problem: hard-to-remove foam is easily generated, and is a hindrance to supply or ejection of ink.
In an attempt to solve this problem, PTL 1 discloses a method of adding a fluorochemical surfactant with a specific structure for the purpose of improving printing quality and ink ejection stability, and a method for improving defoaming quality by employing a specific means of filtration which is performed after the addition of a silicone anti-foaming agent.
Although this method can rectify most of the foaming problem, the use of the silicone anti-foaming agent gives rise to another problem. Specifically, since silicone anti-foaming agents are generally prepared by dispersing fine silica particles into silicone oil, the silicone anti-foaming agents are basically incompatible with aqueous inks and thus easily remain as extraneous matter or residues in a liquid passage of an inkjet recording apparatus, for example. Consequently, there is a serious problem in that ink ejection stability is hindered.
Also, when a conventional fluorochemical surfactant is used, PFOS (perfluorooctanesulfonic acid) or PFOA (perfluorooctanoic acid) will be generated in a production process. It should be noted that the U.S. Environmental Protection Agency (EPA) points out that any compound containing more than four carbon atoms bonded to fluorine atoms is liable to accumulate in human bodies. Therefore, the generation of PFOS or PFOA is problematic in terms of safety.